Large-scale forced drag control in compressible channels

This study explores drag reduction in compressible turbulent channel flows via large-scale streamwise swirls induced by spanwise opposing wall-jet forcing (SOJF). Direct numerical simulations of the compressible Navier–Stokes equations are performed at a bulk Reynolds number of Reb=2800 for both subsonic (Mab=0.8) and supersonic (Mab=1.5) cases. The control parameters are fixed at A+≈0.015 and λ+≈1200, while two different control heights for large-scale swirls are examined, i.e., yc+=10 and 30. We find that SOJF achieves a drag reduction of 14.0% at Mab=0.8 and 6.8% at Mab=1.5 for yc+=30, higher than 8.1% (Ma=0.8) and 0.6% (Ma=1.5) for yc+=10. The better performance of a larger yc+ is found to be caused by a more effective suppression of near-wall velocity streaks and streamwise vortices, associated with a greater reduction of Reynolds shear stress and turbulence production. Moreover, the degraded control efficacy at higher Mach numbers originates from the intrinsically elevated near-wall density in compressible flows, further intensified by SOJF-induced wall-normal ejections. The results indicate that to enhance drag reduction in high-speed flow regimes, it is important to address the density stratification for developing a Mach-number-adaptive control manner.